n.
Production on a cosmic scale of all the chemical element s from one or perhaps two simple types of atomic nuclei (see nucleus ), those of hydrogen and helium .
Elements differ in the number of proton s and isotope s of each element by the number of neutron s in their nuclei. One type of nucleus can be transformed into another by adding or removing protons, neutrons, or both, processes that go on in star s. Many of the first 26 elements (up to iron) and their present cosmic abundances can be accounted for by successive nuclear fusion reactions, beginning with hydrogen , in stellar cores. Heavier elements are believed to be created in the death of stars during supernova explosions, by capture of successive neutrons by lighter nuclei and decay of some of these neutrons into protons (with ejection of an electron and a neutrino each time).